1. Field of the Invention
This invention relates generally to computer peripheral interfaces and more particularly to a logic circuit that adjusts the data clock output of a magnetic stripe reader to the receiver clock input of a programmable communications interface by adding a fixed unit of time to a digital pulse.
2. Description of the Prior Art
An ever-growing array of devices can be attached to a computer system for reading data into a computer. These can be devices by which data is entered by human operators, automatically from such storage devices of magnetic cards. With this type of a device, data stored in a magnetic card can be verified, and used as a form of employee identification. Typically a magnetic card is placed into an appropriate slot and pushed past a reading device which senses and reads the data encoded in the card. Another device for reading data into a computer is a hand-held magnetic wand. With this type of device, the data is encoded on a magnetic stripe deposited on a variety of mediums, such as a paper data card used for inventory in a factory. In order to read this magnetically-coded data, a wand-type reader is swept across the magnetic stripe, and data is transmitted from the wand mechanism to a computer or a communications line. Because each of these devices requires that some sort of data-bearing media be swept across the code-sensing device, this action has a tendency to produce shorter data pulses as the relative velocity increases. However, in order to interface the data clock output of a magnetic swipe reader or wand reader to the receiver clock input of a programmable communications interface for transmission to the computer, and not violate its timing specifications, it is necessary to provide a stable circuit that adds a fixed unit of time to a digital pulse.
In order to solve this problem, prior art methods utilized a combination of a monostable, multi-vibrator (one-shot), and a "D" type latch (flip-flop) along with interconnecting logic requiring resistor capacitor network for the one-shot. This type of solution had the disadvantage of having some time-out variations because of the effects of temperature, component tolerance and variations amongst one-shot parameters.
What is needed is a simple, low-cost and precise digital pulse stretcher which can interface with the data clock output of the magnetic swipe and wand-type readers to the receiver clock input of a programmable communications interface without violating its timing specification.